There are numerous circumstances in which it is desirable to detect, measure or monitor a constituent of a fluid. These include the detection and/or determination of trace metal ions and other species which may occur in aqueous fluids which may for example be a water supply or an effluent.
A particularly challenging context is the analysis of downhole fluids, which can be an important aspect of determining the quality and economic value of a hydrocarbon formation. Knowledge of downhole formation (produced) water chemistry can be applied to save costs and increase production at all stages of oil and gas exploration and production. Measurements obtained downhole can be important for a number of key processes of hydrocarbon production, including:                Prediction and assessment of mineral scale and corrosion;        Strategy for oil/water separation and water re-injection;        Understanding of reservoir compartmentalization/flow units;        Characterization of water break-through;        Derivation of the water cut Rw; and        Evaluation of downhole H2S partition in the oil and or water (if used for H2S measurements).        
Analysis downhole can be valuable because some species contained within a reservoir fluid may be responsible for disadvantageous properties of the fluid, including potential to cause damage to well apparatus that comes into contact with that fluid. Some species within a reservoir fluid, for example heavy metal ions, may potentially have harmful environmental impact if brought to the surface. Analysis of fluids is also relevant to determining the chemistry of a waterbody which may be surface water, groundwater or water in an aquifer. Here too there can be a requirement to detect or determine metal ions.
There have been proposals for electrochemical analytical sensors to be used downhole. Examples include sensors focused on the detection of H2S, pH or the group II scaling ions: Ba, Ca and Sr. See, for example, GB2391314; GB2409902; GB2430749; GB2395555 and GB2404252. Some electrochemical sensors have been directed to the monitoring of trace metals in natural waters. Zirino et al have developed an automated flow system for the continuous measurement of trace metals in sea-water by a method based on anodic stripping voltammetry. U.S. Pat. No. 5,296,123, U.S. Pat. No. 5,437,772, U.S. Pat. No. 5,676,820 and US2001/0042693 disclose sensors for quantification of trace metals (Cd, Cu, Fe, Pb, Ni and Zn) in water and effluent.
In GB2404252 above the sensor includes a ligand which is able to bind to scaling ions and has an electronic configuration which is altered on binding of a scaling ion. The electroactivity of the ligand is then monitored to detect whether a change in electroactivity, which denotes binding by a scaling ion, has taken place. The ligand may be in solution, confined by a membrane, or may be immobilised on conducting particles attached to an electrode.
It has been known for a number of years that the binding of an ion to a ligand can be observed as a change in electrochemical properties and moreover that the change can be observed as a change in electrochemical properties of a redox system adjacent to the bound ligand. This has been demonstrated with ligands bound to metallocenes as redox system. See papers by Plenio et al, by Beer and by Medina et al in the list of references.
The ligands which have been employed have often been macrocyclic compounds such as crown ethers. Preparation of the ligand has sometimes entailed the elaboration of a multi-step synthesis. The electrochemistry has generally been carried out with the metallocene-ligand compound, in solution and its complex with a metal ion likewise in solution.